Sounds very much like Stingray ....... only 3 decades off, but like Thunderbirds it was repeated forever.
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iandotkelly♦Oct 2 '13 at 20:52

Was live action, definitely not stingray I'm afraid: although this now raises interesting questions about the derivation of the show I'm thinking of!
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John Smith OptionalOct 2 '13 at 20:58

Mmm - now I'm puzzled. Do you remember the channel it was on?
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iandotkelly♦Oct 2 '13 at 21:05

More than likely it was BBC2 in the UK, as they used to schedule hour upon hour of kids television during the School Summer Break. The more I think about it though, the more I'm sure it was an Australian show. It had reasonably good production values, and the Mermaid-Girl thing was a babe.
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John Smith OptionalOct 2 '13 at 21:10

2 Answers
2

Could it be Australia's Ocean Girl (released as Ocean Odyssey in the UK)? It featured a character named Neri:

Neri — the title character — is a young girl with an affinity for
water, super-human strength, the ability to swim long distances, and
super-human lung capacity. She lives alone on an otherwise deserted
island. Early in the first season, Neri befriends two Australian boys:
Jason and Brett Bates. This friendship is at first a highly-guarded
secret due to Neri's fear of other humans. The Bates brothers live in
an elaborate underwater research and environmental protection facility
called ORCA, located near Port Douglas, Queensland. At the beginning
of the series, they move to ORCA in order for their mother, Dr. Dianne
Bates, to study whale song. A significant portion of the series takes
place on ORCA itself, and looks at the activities of its inhabitants,
many of whom are school-aged children, just as Jason and Brett.
(Wikipedia)

It ran from 1994-97, and a follow-up version ran in the early 2000s. Neri was played by Marzena Godecki:

A quick Google search suggests Ocean Girl - it's an Australian TV show made between 1994 and 1997 which features a girl named Neri with "super-human lung capacity" who befriends some boys living in an underwater facility.